In a power supply device using an electric power conversion circuit, as a method for inhibiting an inrush current to a smoothing capacitor that is caused by a voltage difference between a power supply system and the smoothing capacitor, there is known a method in which the electric power conversion circuit is bypassed such that the inrush current does not flow therethrough (for example, Japanese Patent Application Laid-Open No. 2011-135758 (JP 2011-135758 A)).
FIG. 1 shows a block diagram of a conventional electric power converter. A power supply system 60 is an alternating-current (AC) power supply to supply AC power to an electric power conversion circuit 40 through reactors L1 and L2. Direct-current (DC) power, being an output of the electric power conversion circuit 40, is fed to a smoothing capacitor 70, and smoothed electric power is fed to a load 50.
In the above conventional technique, when a voltage difference occurs between the power supply system 60 and the smoothing capacitor 70, a switch SW1 connected to the electric power conversion circuit 40 is first turned off. Thus, an inrush current flows through diodes D1 to D4 and hence does not flow into the electric power conversion circuit 40. However, the diodes D1 to D4 that can withstand the inrush current have to be prepared. Additionally, the power supply system 60 of three-phase AC requires six diodes in total, resulting in cost and size problems.
There is known a power supply device using an electric power conversion circuit that has a plurality of power supply systems for use in an emergency, other than a main power supply system, in order to continue supplying electric power to a load in the event of a power failure. FIG. 2 shows a block diagram of a conventional power supply device. An n number of power supply systems in total, i.e. a first power supply system 60-1, a second power supply system 60-2, . . . , and an n-th power supply system 60-n are connected in parallel as the power supply systems. A power failure detection circuit 10 is connected to each of the n number of power supply systems. The power failure detection circuit 10 detects the presence or absence of a power failure in each of the n number of power supply systems. A first charging circuit 30-1, a second charging circuit 30-2, . . . , and an n-th charging circuit 30-n are connected to the first power supply system 60-1, the second power supply system 60-2, . . . , and the n-th power supply system 60-n, respectively. Furthermore, outputs of the first charging circuit 30-1, the second charging circuit 30-2, . . . , and the n-th charging circuit 30-n are connected to contact points x, y, . . . , z of a changing switch 20, respectively.
The power failure detection circuit 10 detects the presence or absence of a power failure in the n number of power supply systems, and switches the power supply system changing switch 20 based on detection results. By way of example, the changing switch 20 first connects the contact point x on an input side to a terminal OUT20 on an output side to feed electric power from the first power supply system 60-1 to the electric power conversion circuit 40. At this time, if the power failure detection circuit 10 detects the occurrence of a power failure in the first power supply system 60-1, the power failure detection circuit 10 controls the power supply system changing switch 20, such that the contact point y on the input side is connected to the terminal OUT20 on the output side and electric power is fed from the second power supply system 60-2. In a like manner, when the power failure detection circuit 10 detects the occurrence of a power failure in the second power supply system 60-2, the power failure detection circuit 10 controls the power supply system changing switch 20, such that the contact point z on the input side is connected to the terminal OUT20 on the output side and electric power is fed from the n-th power supply system 60-n (n 3).
In the conventional power supply device shown in FIG. 2, in switching from a power supply system having a power failure to a normal power supply system having no power failure, there is a delay in a switching operation of the power supply system changing switch 20. Thus, there is a case in which a smoothing capacitor 41 in the electric power conversion circuit 40 is discharged and a potential difference occurs between the power supply system and the smoothing capacitor 41. At this time, switching the power supply systems in a state of having the potential difference causes a flow of an inrush current through the smoothing capacitor 41 and damages a switching element and the like in the electric power conversion circuit 40.
The inrush current flowing through the smoothing capacitor 41 upon the switching of the power supply systems will be described with reference to a timing chart of FIG. 3. FIG. 3 shows the variation in voltage of the smoothing capacitor 41 and the inrush current to the smoothing capacitor 41 with time when switching the power supply systems due a power failure. First, the electric power is fed from the first power supply system 60-1 until time t10, so that the smoothing capacitor 41 in the electric power conversion circuit 40 is charged with a voltage V0. Then, if the power failure occurs at the time t10, the voltage of the smoothing capacitor 41 is gradually discharged and decreases with time.
As shown in FIG. 2, when a power failure occurs in the first power supply system 60-1, the power failure detection circuit 10 detects the power failure and controls the power supply system changing switch 20 so as to select the second power supply system 60-2. However, since it takes a predetermined amount of time from the detection of the power failure to complete the switching of the changing switch 20, the voltage of the smoothing capacitor 41 decreases to V1 until time t11 at the point in time of completing the switching to the second power supply system 60-2. In switching from the first power supply system 60-1 to the second power supply system 60-2 at the time t11, the smoothing capacitor 41 is charged in accordance with the magnitude of the potential difference (=V0−V1), which is the decrease in the voltage of the smoothing capacitor 41 and hence the inrush current flows through the smoothing capacitor 41. At this time, provided that the charging of the smoothing capacitor 41 is completed at time t12, the shorter the time between the time t11 of switching to the second power supply system 60-2 and the time t12, the larger the inrush current flowing through the smoothing capacitor 41. T0max represents a maximum value of the inrush current.
As shown in FIG. 2, there is known a method in general in which the charging circuits 30-1, 30-2, . . . , 30-n are provided in series to the power supply systems 60-1, 60-2, . . . , 60-n, respectively, to inhibit the inrush current in switching the power supply systems. However, it is necessary to provide the charging circuit for each individual power supply system, resulting in cost and size problems.
Also, as shown in FIG. 4, the electric power is fed continuously through the charging circuit, even after the power supply systems are switched from the first power supply system 60-1 having the power failure to the normal second power supply system 60-2 having no power failure and the second charging circuit 30-2 inhibits the inrush current to the smoothing capacitor 41. Therefore, parts such as current-limiting resistor or inductance, which constitute the charging circuit, bring about a voltage drop across the power supply system and degrade electric power conversion efficiency. Moreover, heat generated by the parts themselves affects the life of elements and peripheral circuits.
An object of the present invention is to provide an electric power converter that can switch power supply systems in a stable manner in the event of a power failure, with preventing an increase in the number of parts, a voltage drop across the power supply system, and an increase in heat generated in the electric power converter.